Process For Recycling Polyacetals

ABSTRACT

A process for recycling polyoxymethylene polymers is disclosed. A polyoxymethylene polymer is at least partially dissolved in an aprotic compound. The resulting solution or suspension (liquid mixture) is then contacted with a catalyst which causes the polyoxymethylene polymer to be converted into a cyclic acetal. The cyclic acetal can be separated, collected and used in other processes. In one embodiment, the cyclic acetal may be used to produce a polyoxymethylene polymer.

BACKGROUND

Oxymethylene polymers, which include polyoxymethylene homopolymers andpolyoxymethylene copolymers, possess many useful properties andcharacteristics. For example, the polymers can have great strengthproperties while also being chemical resistant. The polymers can also beeasily molded into any desired shape. The polymers are currently beingused in all different types of applications. For instance,polyoxymethylene polymers are being used to form interior or exteriorautomotive parts, parts for consumer appliances, parts for industrialprocesses, and the like.

Oxymethylene polymers can be produced via anionic polymerization ofanhydrous formaldehyde or can be produced through the cationicpolymerization of formaldehyde or cyclic oligomers, such as trioxane.During cationic polymerization, the polymer can be formed in bulk (i.e.without solvent). Alternatively, the polymerization can take place insolution where the polymer precipitates in a solvent to form aheterogeneous phase. In still another embodiment, a majority of thepolymer may be formed in the heterogeneous phase followed by furtherpolymerization in a homogeneous phase.

During the formation of oxymethylene polymers, cationic initiators aretypically combined with one or more monomers to initiate polymerization.After polymerization, the reaction mixture can be rapidly and completelydeactivated by adding a deactivator.

The deactivator can be added to a heterogeneous phase after the polymerhas precipitated in a solvent, or can occur during a homogeneous phase,while the polymer is in a melted form. After being deactivated, theresultant polymer can be ground and/or pelletized. In some embodiments,the polyoxymethylene polymer is compounded with various differentcomponents in order to produce a master batch. The master batch can thenbe combined with greater amounts of polyoxymethylene polymer resin orother ingredients during a molding process to produce various products.

During the production of polyoxymethylene polymers, some scrap materialis produced that is comprised primarily of the polyoxymethylene polymer.Since recycling of the waste products coming from production andcompounding of the polymer is beneficial, in the past, various differentprocesses have been proposed in order to recycle the polymer. In oneembodiment, for instance, the reclaimed polyoxymethylene polymer isconverted into an aqueous solution of formaldehyde. The aqueous solutionof formaldehyde can then be used to produce various products, such astrioxane. Recycling polyoxymethylene polymers in this manner, however,is not cost effective. In addition, conversion of the polymer intotrioxane is relatively low.

In view of the above, a need currently exists for a process of recyclingpolyoxymethylene polymers. In particular, a need exists for a processfor recycling polyoxymethylene polymers and converting them into usablemonomers, such as a cyclic acetal.

SUMMARY

In general, the present disclosure is directed to a process forrecycling polyoxymethylene polymers. The present disclosure is alsodirected to a process for producing a cyclic acetal.

In one embodiment, for instance, the present disclosure is directed to aprocess for the conversion of oxymethylene homo- or copolymers to cyclicacetals comprising the steps:

a) at least partly dissolving an oxymethylene homo- or copolymer in anliquid medium comprising or consisting of an aprotic compound and

b) converting the at least partly dissolved oxymethylene homo- orcopolymer in the presence of a catalyst to cyclic acetals.

The present disclosure is also directed to a process for the recyclingof oxymethylene homo- or copolymers comprising the steps:

a) at least partly dissolving an oxymethylene homo- or copolymer in anaprotic compound;

b) converting the at least partly dissolved oxymethylene homo- orcopolymer in the presence of a catalyst to cyclic acetals; and

c) converting the cyclic acetals obtained in step b) optionally togetherwith comonomer(s) to oxymethylene polymers.

In one particular embodiment, the process for producing a cyclic acetal,preferably trioxane and/or tetroxane, comprises:

preparing a liquid reaction mixture comprising:

-   -   a) 5-70 wt.-%, preferably 20-70 wt.-%, more preferably 30-60        wt.-% of a polyoxymethylene homo- or copolymer;    -   b) 25-90 wt.-%, preferably 25-70 wt.-%, more preferably 30-65        wt.-% of an aprotic compound;    -   c) 0.001 to 10 wt.-% of a catalyst; and    -   d) optionally less than 20 wt.-% of water, wherein the amounts        are based on the total weight of the reaction mixture;

converting the polyoxymethylene homo- or copolymer into a cyclic acetal.

Typically, the reaction is carried out at a temperature higher thanabout 0° C., preferably ranging from about 0° C. to about 150° C., morepreferably ranging from about 10° C. to about 120° C., furtherpreferably from about 20° C. to about 100° C. and most preferably fromabout 30° C. to about 90° C.

A further advantage of the process of the present invention is that thecyclic acetals can easily be separated from the reaction mixture. Thecyclic acetal, especially the trioxane can be separated from thereaction mixture by distillation in a high purity grade. Especially incase aprotic compounds (such as sulfolane) having a boiling point higherthan about 20° C. above the boiling point of the cyclic acetals are usedthe formed cyclic acetals can simply be distilled off. In case sulfolaneis used as the aprotic compound the formed trioxane can be distilled offwithout the formation of an azeotrope of sulfolane with trioxane. Theprocess of the invention can be carried out batch wise or as acontinuous process.

In a preferred embodiment the process is carried out as a continuousprocess wherein the polyoxymethylene polymer is continuously fed to theliquid medium comprising the catalyst and wherein the cyclic acetals,e.g. the trioxane, is continuously separated by separation methods suchas distillation.

The process of the invention leads to a lower energy consumption andlower costs for the separation of the cyclic acetals. Due to the highconversion of the polyoxymethylene polymer to the desired cyclic acetalssaid cyclic acetals can be much more efficiently produced.

According to a preferred embodiment the final conversion of thepolyoxymethylene polymer to the cyclic acetal is greater than 10%.

The final conversion refers to the conversion of the polymer into thecyclic acetals in the liquid system. The final conversion corresponds tothe maximum conversion achieved in the liquid system.

The final conversion of the polyoxymethylene polymer to the cyclicacetals can be calculated by dividing the amount of cyclic acetals(expressed in wt.-%, based on the total weight of the reaction mixture)in the reaction mixture at the end of the reaction divided by the amountof polymer (expressed in wt.-%, based on the total weight of thereaction mixture) at the beginning of the reaction at t=0.

For example the final conversion can be calculated as:

Final conversion=(amount of trioxane in the reaction mixture expressedin weight-% at the end of the reaction)/(amount of polyoxymethylenepolymer in the reaction mixture expressed in weight-% at t=0 [initialamount of polymer in the reaction mixture])

According to a further preferred embodiment of the process of theinvention the final conversion of the polymer into the cyclic acetals,preferably trioxane and/or tetroxane, is higher than 12%, preferablyhigher than 14%, more preferably higher than 16%, further preferablyhigher than 20%, especially higher than 30%, particularly higher than50%, for example higher than 80% or higher than 90%.

A further embodiment is a process for producing cyclic acetal comprising

i) preparing a liquid mixture comprising

-   -   a) a polyoxymethylene polymer and    -   b) an aprotic compound;        ii) contacting the liquid mixture with a catalyst; and        iii) converting the polyoxymethylene polymer into cyclic acetal,        wherein the final conversion of the polymer to said cyclic        acetal is greater than 10% on basis of the initial polymer.

According to this preferred embodiment of the present invention a liquidmixture as defined above can be prepared and contacted with a catalystas defined above. According to a preferred embodiment the catalyst is asolid catalyst which at least remain partly solid under the reactionconditions. Preferably the catalyst is selected from fixed bed catalyst,acid ion-exchange material and solid support carrying Bronsted and/orLewis acids. Alternatively, the catalyst can be a liquid catalyst whichis only partly miscible or essentially immiscible with liquid mixture.

The aprotic compound used in the process may be polar. For instance, inone embodiment, the aprotic compound may be dipolar. In one embodiment,the aprotic compound comprises a sulfur containing organic compound suchas a sulfoxide, a sulfone, a sulfonate ester, or mixtures thereof. Inone embodiment, the aprotic compound comprises sulfolane.

The aprotic compound may also have a relatively high static permittivityor dielectric constant of greater than about 15. The aprotic compoundmay also be nitro-group free. In particular, compounds havingnitro-groups may form undesired side reactions within the process.

Other features and aspects of the present disclosure are discussed ingreater detail below.

DETAILED DESCRIPTION

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentdisclosure.

In general, the present disclosure is directed to a process forproducing one or more cyclic acetals. The process of the presentdisclosure, for instance, may be used to recycle polyoxymethylenepolymers. In particular, according to the present disclosure, apolyoxymethylene polymer is at least partly dissolved in an aproticsolvent. For instance, the polymer can be dissolved in the solvent toform a solution or to form a suspension. In one embodiment, the polymeris dissolved in the solvent at elevated temperatures.

Upon cooling, before the polymer precipitates, a catalyst is contactedwith the solution or suspension. In a relatively short amount of time,such as within minutes, the polyoxymethylene polymer can be almostcompletely converted to a cyclic acetal, such as trioxane and/ortetroxane. The one or more cyclic acetals can then be isolated byevaporation and can be purified through distillation.

In one embodiment, the cyclic acetals are then used to produce furtheramounts of a polyoxymethylene polymer.

The process of the present disclosure includes many benefits andadvantages. For instance, the process can be designed to be highlyefficient. For instance, greater than 10%, such as greater than 20%,such as greater than 40%, such as greater than 60%, such as greater than70%, such as even greater than 90% of the polyoxymethylene polymer maybe converted into a cyclic acetal. Further, as mentioned above, theprocess can occur very rapidly.

As described above, the process can be used to recycle polyoxymethylenepolymers that are reclaimed during the production process or duringcompounding. It should be understood, however, that polyoxymethylenepolymers collected from the solid waste stream can also be processedaccording to the present disclosure.

As described above, the polyoxymethylene polymer is contacted with acatalyst in the presence of an aprotic compound to form a cyclic acetal.The polyoxymethylene polymer may comprise a homopolymer or a copolymer.In one embodiment, the polyoxymethylene homo- and/or copolymer has anumber average molecular weight (Mn) of more than 2,000 Dalton.

The aprotic compound or solvent provides various advantages to theprocess. For example, not only is the aprotic compound a solvent for thepolymer, but the aprotic compound also facilitates production of thecyclic acetal in a manner that greatly enhances conversion rates.

Of particular advantage, the cyclic acetal produced according to theprocess can then be easily separated from the aprotic compound and thecatalyst. For instance, in one embodiment, the cyclic acetal can beseparated or isolated from the aprotic compound through a simpledistillation process, since the aprotic compound may have a much higherboiling point than the cyclic acetal.

In one embodiment, the aprotic compound is a liquid when contacted withthe polyoxymethylene polymer. The polyoxymethylene polymer may dissolveinto the aprotic compound or may be depolymerized in the aproticcompound to form a homogeneous phase. The aprotic compound and thecatalyst, in one embodiment, may comprise a liquid reaction mixture or aliquid medium.

An advantage of the present invention is that the conversion of thepolyoxymethylene polymer can be carried out in a liquid system, e.g., aliquid reaction mixture or a liquid medium or a liquid mixture. However,even though it is advantageous, the components of the reaction mixtureor the liquid mixture or the liquid medium must not necessarilycompletely be dissolved. Thus, the reaction mixture or the liquidmixture or liquid medium may also comprise solids or molten componentswhich are not dissolved.

The polyoxymethylene polymer reacts (converts) in the presence of acatalyst. Usually, cationic catalysts, such as Bronsted acids or Lewisacids, accelerate the conversion of the polyoxymethylene polymer to thedesired cyclic acetals.

The catalyst is a catalyst for the conversion (reaction) of apolyoxymethylene polymer into cyclic acetals, in particular intotrioxane and/or tetroxane.

Cyclic acetals within the meaning of the present disclosure relate tocyclic acetals derived from formaldehyde. Typical representatives arerepresented the following formula:

wherein a is an integer ranging from 1 to 3.

Preferably, the cyclic acetals produced by the process of the presentdisclosure are trioxane (a=1) and/or tetroxane (a=2). Trioxane andtetroxane usually form the major part (at least 80 wt.-%, preferably atleast 90 wt.-%) of the cyclic acetals formed by the process of thepresent disclosure.

The weight ratio of trioxane to tetroxane varies with the catalyst used.Typically, the weight ratio of trioxane to tetroxane ranges from about3:1 to about 40:1, preferably about 4:1 to about 20:1.

As used herein, an aprotic compound is a compound that does not containany substantial amounts of hydrogen atoms which can disassociate.

In one embodiment, the aprotic compound is liquid under the reactionconditions. Therefore, the aprotic compound may have a melting point ofabout 180° C. or less, preferably about 150° C. or less, more preferablyabout 120° C. or less, especially about 60° C. or less.

For practical reasons, it is advantageous to use an aprotic compoundwhich has a melting point in the order of preference (the lower themelting point the more preferred) of below about 50° C., below about 40°C. and below about 30° C. and below about 20° C. Especially, aproticcompounds which are liquid at about 25 or about 30° C. are suitablesince they can be easily transported by pumps within the productionplant.

Further, the aprotic compound may have a boiling point of about 120° C.or higher, preferably about 140° C. or higher, more preferably about160° C. or higher, especially about 180° C. or higher, determined at 1bar. In a further embodiment the boiling point of the aprotic compoundis about 200° C. or higher, preferably about 230° C. or higher, morepreferably about 240° C. or higher, further preferably about 250° C. orhigher and especially about 260° C. or higher or 270° C. or higher. Thehigher the boiling point the better the cyclic acetals, especiallytrioxane and/or tetroxane, formed by the process of the presentdisclosure can be separated by distillation. Therefore, according to anespecially preferred embodiment of the present disclosure the boilingpoint of the aprotic compound is at least about 20° C. higher than theboiling point of the cyclic acetal formed, in particular at least about20° C. higher than the boiling point of trioxane and/or tetroxane.

Additionally, aprotic compounds are preferred which do not form anazeotrope with the cyclic acetal, especially do not form an azeotropewith trioxane.

In a preferred embodiment of the present invention the reaction mixtureor liquid medium in the reactor 40 comprises at least about 20 wt.-%,preferably at least about 40 wt.-%, more preferably at least about 60wt.-%, most preferably at least about 80 wt.-% and especially at leastabout 90 wt.-% of the aprotic compound(s), wherein the weight is basedon the total weight of the reaction mixture. The liquid medium or thereaction mixture or the liquid mixture may comprise one or more aproticcompound(s).

In a preferred embodiment the liquid medium is essentially consisting ofthe aprotic compound. Essentially consisting of means that the liquidmedium comprises at least about 95 wt.-%, preferably at least about 98wt.-%, more preferably at least about 99 wt.-%, especially at leastabout 99.5 wt.-%, in particular at least about 99.9 wt.-% of the aproticcompound(s). In a further embodiment of the invention the liquid mediumis the aprotic compound, i.e., the liquid medium is consisting of theaprotic compound.

It has been found that liquid aprotic compounds which at least partlydissolve or depolymerized the polyoxymethylene polymer lead to excellentresults in terms of conversion of the polyoxymethylene polymer into thedesired cyclic acetals.

Therefore, aprotic compounds are preferred which at least partlydissolve or depolymerized the polyoxymethylene polymer under thereaction conditions.

The aprotic compound used in the process can be a polar aproticcompound, especially a dipolar compound. Polar aprotic solvents are muchmore suitable to dissolve the polyoxymethylene polymer. Non-polaraprotic compounds such as unsubstituted hydrocarbons (e.g. cyclichydrocarbons such as cyclohexane, or alicyclic hydrocarbons such ashexane, octane, decane, etc.) or unsubstituted unsaturated hydrocarbonsor unsubstituted aromatic compounds are less suitable. Therefore,according to a preferred embodiment the aprotic compound is not anunsubstituted hydrocarbon or unsubstituted unsaturated hydrocarbon orunsubstituted aromatic compound. Further, preferably the reactionmixture comprises unsubstituted hydrocarbons and/or unsubstitutedunsaturated hydrocarbons and/or unsubstituted aromatic compounds in anamount of less than about 50 wt.-%, more preferably less than about 25wt.-%, further preferably less than about 10 wt.-%, especially less thanabout 5 wt.-%, e.g. less than about 1 wt.-% or about 0 wt.-%.

Halogen containing compounds are less preferred due to environmentalaspects and due to their limited capability to dissolve thepolyoxymethylene polymer. Further, the halogenated aliphatic compoundsmay cause corrosion in vessels or pipes of the plant and it is difficultto separate the cyclic acetals formed from the halogenated compounds.

According to one embodiment, the aprotic compound is halogen free. In afurther preferred embodiment the reaction mixture comprises less thanabout 50 wt.-%, more preferably less than about 25 wt.-%, furtherpreferably less than 10 wt.-%, more preferably less than 5 wt.-%,especially less than 1 wt.-% or 0 wt.-% of halogenated compounds.

Likewise, the use of (liquid) sulphur dioxide leads to difficulties withisolation of the cyclic acetals. Therefore, the aprotic compound ispreferably free of sulphur dioxide. In a further preferred embodimentthe reaction mixture comprises less than about 50 wt.-%, more preferablyless than about 25 wt.-%, further preferably less than 10 wt.-%, morepreferably less than 5 wt.-%, especially less than 1 wt.-% or 0 wt.-% ofsulphur dioxide.

Polar aprotic compounds are especially preferred. According to apreferred embodiment of the invention the aprotic compound has arelative static permittivity of more than about 15, preferably more thanabout 16 or more than about 17, further preferably more than about 20,more preferably of more than about 25, especially of more than about 30,determined at 25° C. or in case the aprotic compound has a melting pointhigher than 25° C. the relative permittivity is determined at themelting point of the aprotic compound.

The relative static permittivity, ∈_(r), can be measured for staticelectric fields as follows: first the capacitance of a test capacitorC₀, is measured with vacuum between its plates. Then, using the samecapacitor and distance between its plates the capacitance C_(x) with anaprotic compound between the plates is measured. The relative dielectricconstant can be then calculated as

$ɛ_{r} = {\frac{C_{x}}{C_{0}}.}$

Within the meaning of the present invention the relative permittivity isdetermined at 25° C. or in case the aprotic compound has a melting pointhigher than 25° C. the relative permittivity is determined at themelting point of the aprotic compound.

Preferred are aprotic compounds which dissolve the polyoxymethylenepolymer. According to a preferred embodiment the polyoxymethylenepolymer is at least partially, preferably at least about 80 wt.-%, morepreferably at least about 95 wt.-%, especially completely, in solutionin the reaction mixture or liquid mixture.

According to a further aspect of the invention the aprotic compound is adipolar aprotic compound.

The aprotic compound within the meaning of the present invention isgenerally a dipolar and non-protogenic compound which has a relativepermittivity as defined above of more than 15, preferably more than 25or more than 30, determined at 25° C. or in case the aprotic compoundhas a melting point higher than 25° C. the relative permittivity isdetermined at the melting point of the aprotic compound.

The process can be carried out in manner wherein the polyoxymethylenepolymer is completely dissolved or absorbed in the liquid medium orreaction mixture or liquid mixture. Therefore, according to oneembodiment the polyoxymethylene polymer and the aprotic compound form ahomogenous phase under the reaction conditions.

In one embodiment, the polyoxymethylene polymer and the aprotic compoundare combined together and heated in order to dissolve a substantialportion of the polymer. For instance, the aprotic solvent and polymercan be heated to a temperature of greater than about 130° C., such asgreater than about 140° C., such as greater than about 150° C., such asgreater than about 160° C., such as greater than about 170° C., such asgreater than about 180° C., such as greater than about 190° C. Thetemperature to which the mixture is heated depends in part on theboiling point of the aprotic compound. In one embodiment, for instance,the aprotic compound and polymer are heated to a temperature of fromabout 150° C. to about 200° C., such as from about 160° C. to about 180°C.

Suitable aprotic compounds are selected from the group consisting oforganic suffoxides, organic sulfones, organic sulfonate ester, andmixtures thereof.

According to a preferred embodiment the aprotic compound is selectedfrom sulfur containing organic compounds.

Further, the aprotic compound is preferably selected from the groupconsisting of cyclic or alicyclic organic sulfoxides, alicyclic orcyclic sulfones, and mixtures thereof.

Excellent results can be achieved by aprotic compounds as represented bythe following formula (I):

whereinn is an integer ranging from 1 to 6, preferably 2 or 3, andwherein the ring carbon atoms may optionally be substituted by one ormore substituents, preferably selected from C₁-C₈-alkyl which may bebranched or unbranched. Preferred compounds of formula (I) are sufolane,methylsuffolane, dimethylsulfolane, ethylsulfolane, diethylsulfolane,propylsulfolane, dipropylsulfolane, butylsulfolane, dibutylsulfolane,pentylsulfolane, dipentylsulfolane, and hexylsulfolane as well asoctylsulfolane.

According to the most preferred embodiment the aprotic compound issulfolane (tetrahydrothiophene-1,1-dioxide).

Sulfolane is an excellent solvent for the polyoxymethylene polymer, itis stable under acidic conditions, it does not deactivate the catalystsand it does not form an azeotrope with trioxane. Further, it is asolvent which is inert under the reaction conditions.

Unless indicated otherwise the expression “reaction mixture” refers tothe mixture which is used for the reaction of the polyoxymethylenepolymer to the cyclic acetals. The concentrations and amounts of theindividual components of the reaction mixture refer to theconcentrations and amounts at the beginning of the reaction. In otherwords the reaction mixture is defined by the amounts of its startingmaterials, i.e. the amounts of initial components.

Likewise the amounts defined for the “liquid mixture” refer to theamounts of the components at the beginning of the reaction, i.e. priorto the reaction.

The polyoxymethylene polymer reacts to the cyclic acetals and, as aconsequence, the concentration of the polyoxymethylene polymer decreaseswhile the concentration of the cyclic acetals increases.

At the beginning of the reaction a typical reaction mixture of theinvention comprises a polyoxymethylene polymer which is at least partly,preferably completely dissolved or absorbed in sulfolane and a catalyst.

Further, an especially preferred embodiment of the present invention isa process for producing cyclic acetal comprising reacting apolyoxymethylene polymer in the presence of a catalyst wherein thereaction is carried out in sulfolane or a process for producing cyclicacetal from a polyoxymethylene polymer in the presence of a catalyst andsulfolane.

A further preferred aprotic compound is represented by formula (II):

wherein R¹ and R² are independently selected from C₁-C₈-alkyl which maybe branched or unbranched, preferably wherein R¹ and R² independentlyrepresent methyl or ethyl. Especially preferred is dimethyl sulfone.

According to a further preferred embodiment the aprotic compound isrepresented by formula (III):

whereinn is an integer ranging from 1 to 6, preferably 2 or 3, andwherein the ring carbon atoms may optionally be substituted by one ormore substituents, preferably selected from C₁-C₈-alkyl which may bebranched or unbranched.

Suitable aprotic compounds are also represented by formula (IV):

wherein R³ and R⁴ are independently selected from C₁-C₈-alkyl which maybe branched or unbranched, preferably wherein R¹ and R² independentlyrepresent methyl or ethyl.

Especially preferred is dimethyl sulfoxide.

Suitable aprotic compounds may be selected from aliphatic dinitriles,preferably adiponitrile.

In a further aspect of the invention a mixture of two or more aproticcompounds is used. A mixture of aprotic compounds may be used todecrease the melting point of the aprotic medium. In a preferredembodiment the aprotic compound comprises or is consisting of a mixtureof sulfolane and dimethyl sulfoxide.

The process of the invention is carried out in the presence of acatalyst for the conversion of the polyoxymethylene polymer into cyclicacetals. Suitable catalysts are any components which accelerate theconversion of the polyoxymethylene polymer to the cyclic acetals.

The catalyst is a catalyst for the conversion (reaction) of apolyoxymethylene polymer into cyclic acetals, preferably into trioxaneand/or tetroxane.

In one embodiment, although not necessary, the liquid mixture or mediumcomprising the aprotic compound and the dissolved polymer can be cooledprior to contacting the catalyst. For instance, in one embodiment, theaprotic compound and polymer may be cooled to a temperature of less thanabout 160° C., such as less than about 150° C., such as less than about140° C., such as less than about 130° C. For instance, in oneembodiment, the liquid reaction mixture may be at a temperature of fromabout 100° C. to about 140° C. when contacted with the catalyst, such asfrom about 115° C. to about 135° C.

Usually, cationic catalysts can be used for the process of theinvention. The formation of cyclic acetals can be heterogeneously orhomogenously catalysed. In case the catalysis is heterogeneous theliquid mixture comprising the polyoxymethylene polymer and the aproticcompound is contacted with the solid catalyst or an immiscible liquidcatalyst. A typical liquid immiscible catalyst is a liquid acidic ionexchange resin. Solid catalyst means that the catalyst is at leastpartly, preferably completely in solid form under the reactionconditions. Typical solid catalysts which may be used for the process ofthe present invention are acid ion-exchange material, Lewis acids and/orBronsted acids fixed on a solid support, wherein the support may be aninorganic material such as SiO₂ or organic material such as organicpolymers.

Preferred catalysts are selected from the group consisting of Bronstedacids and Lewis acids. The catalyst is preferably selected from thegroup consisting of trifluoromethanesulfonic acid, perchloric acid,methanesulfonic acid, toluenesulfonic acid and sulfuric acid, orderivatives thereof such as anhydrides or esters or any otherderivatives that generate the corresponding acid under the reactionconditions. Lewis acids like boron trifluoride, arsenic pentafluoridecan also be used. It is also possible to use mixtures of all theindividual catalysts mentioned above.

The catalyst is typically used in an amount ranging from about 0.001 toabout 15 wt.-%, preferably about 0.01 to about 5 wt.-% or about 0.01 toabout 10 wt.-%, more preferably from about 0.05 to about 2 wt.-% andmost preferably from about 0.05 to about 0.5 wt.-%, based on the totalweight of the reaction mixture.

Advantageously, the aprotic compound does not essentially deactivate thecatalyst. Generally, the catalysts used for the formation of cyclicacetals from a polyoxymethylene polymer are cationic catalysts, such asBronsted acids or Lewis acids. Preferably, under the reaction conditionsthe aprotic compound does essentially not deactivate the catalyst usedin the process of the present invention. Aprotic solvents such asdimethylformamide (DMF), dimethylacetamide (DMAC) or N-methylpyrrolidone(NMP) are too basic and therefore may deactivate the catalyst and, as aconsequence, said solvents are less suitable. According to a preferredembodiment of the present invention the liquid reaction mixture isessentially free of amides, preferably essentially free of acylic orcyclic amides. Essentially free means that the amides may be present inan amount of less than about 5 wt.-%, preferably less than about 2wt.-%, more preferably less than 0.5 wt.-%, especially less than about0.01 wt.-% and, in particular, less than 0.001 wt.-% or about 0 wt.-%,wherein the weight is based on the total weight of the liquid reactionmixture.

Nitro group containing compounds can lead to undesired side products oreven demonstrate an insufficient solubility for the polymers.

Therefore, the aprotic compound preferably does not comprise a nitrogroup and/or a nitrogen atom. Further, according to a preferredembodiment of the present invention the aprotic compound is anon-aromatic aprotic compound. Especially, the aprotic compound is notnitrobenzene or an aromatic nitro compound. Further, preferably, theaprotic compound does not comprise ether.

Within the meaning of the present invention the aprotic compound doesnot deactivate the catalyst if under the reaction conditions less thanabout 95%, preferably less than about 50%, more preferably less thanabout 10%, of the Bronsted acid catalyst used protonates the aproticcompound. In case a Lewis acid catalyst is used the aprotic compounddoes not deactivate the catalyst if under the reaction conditions lessthan about 90 wt.-%, preferably less than about 50 wt.-%, morepreferably less than about 10 wt.-% of the Lewis acid catalyst forms acomplex with the aprotic compound.

The degree of protonation and complex formation can be determined by NMRspectroscopy such as ¹H or ¹³C-NMR. The degree of protonation andcomplex formation is determined at 250° C., preferably in d₆-DMSO.

The deactivation of the catalyst can also be determined in the followingmanner.

10 g of commercially available paraformaldehyde (95 wt. %) is dissolvedin 100 g of sulfolane at a temperature sufficient to dissolve theparaformaldehyde in such a way that no gaseous formaldehyde can escape.The clear solution is kept at 90° C. and 0.1 wt. % of triflic acid isadded. The rate of the formation of trioxane is measured (by measuringthe concentration of trioxane as a function of time).

The same experiment is repeated, except that 10 g of the sulfolane arereplaced by 10 g of the aprotic compound to be tested. If the rate oftrioxane formation is still greater than about 1%, preferably greaterthan about 5%, more preferably greater than about 10%, of the rate ofthe initial experiment then it is concluded that the aprotic compound inquestion does not deactivate the catalyst (even though it may reduce itsactivity).

The aprotic compound should not be too basic in order to avoiddeactivation of the catalysts. On the other hand the aprotic compoundpreferably does not chemically react with the polyoxymethylene polymerunder the reaction conditions, i.e. is an inert aprotic compound.

Preferably, under the reaction conditions the aprotic compound shouldnot react chemically with the polyoxymethylene polymer or the cyclicacetal obtained by the process of the invention. Compounds like waterand alcohols are not suitable as they react with formaldehyde. Withinthe meaning of the present invention an aprotic compound does notchemically react with the polyoxymethylene polymer when it meets thefollowing test criteria:

5 g of commercially available paraformaldehyde (95 wt.-%) is added to100 g of the aprotic compound containing 0.1 wt.-%trifluoromethanesulfonic acid and heated at 120° C. for 1 hour withstirring in a closed vessel so that no gaseous formaldehyde can escape.If less than about 1 wt.-%, preferably less than about 0.5 wt.-%, morepreferably less than about 0.1 wt.-% and most preferably less than about0.01 wt.-% of the aprotic compound has chemically reacted, then theaprotic compound is considered not to have reacted with thepolyoxymethylene polymer. If the aprotic compound meets the criteria itis considered inert.

Further, under the acidic reaction conditions the aprotic compoundshould be essentially stable. Therefore, aliphatic ethers or acetals areless suitable as aprotic compounds. The aprotic compound is consideredstable under acidic conditions within the meaning of the presentinvention if the aprotic compound meets the following test conditions:

100 g of the aprotic compound to be tested containing 0.5% by weight(wt.-%) trifluoromethanesulfonic acid is heated at 120° C. for 1 hour.If less than about 0.5 wt.-%, preferably less than about 0.05 wt.-%,more preferably less than about 0.01 wt.-% and most preferably less thanabout 0.001 wt.-% of the aprotic compound has chemically reacted, thenthe aprotic compound is considered to be stable under acidic conditions.

Preferably, the reaction mixture comprises the polyoxymethylene polymerin an amount ranging from about 0.1 to about 80 wt.-% or about 1 to lessthan about 80 wt.-%, more preferably from about 5 to about 75 wt.-%,further preferably ranging from about 10 to about 70 wt.-% and mostpreferred ranging from about 20 to about 70 wt.-%, especially rangingfrom 30 to 60 wt.-% based on the total weight of the reaction mixture.

It has been found that especially good results in terms of conversioncan be achieved when the polyoxymethylene polymer is dissolved in a highconcentration in the aprotic compound.

Therefore, in a further aspect the amount of polyoxymethylene polymer isat least 5 wt.-% or at least 10 wt.-%, preferably ranging from 5 to 75wt.-%, further preferably 10 to 70 wt.-%, especially 15 to 60 wt.-%,based on the total weight of the homogeneous or heterogeneous liquidmixture consisting of the polyoxymethylene polymer and the aproticcompound.

According to a preferred embodiment the weight ratio of polyoxymethylenepolymer to aprotic compound is ranging from about 1:1000 to about 4:1,preferably about 1:600 to about 3:1, more preferably about 1:400 toabout 2:1, further preferably about 1:200 to about 1:1, especiallypreferably about 1:100 to about 1:2, particularly about 1:50 to about1:3, for example about 1:20 to about 1:6 or about 1:15 to about 1:8.

Typically, the reaction is carried out at a temperature higher thanabout 0° C., preferably ranging from about 0° C. to about 150° C., morepreferably ranging from about 10° C. to about 120° C.

The pressure during the reaction can generally be from about 10millibars to about 20 bars, such as from about 0.5 bar to about 10 bar,such as from about 0.5 bar to about 2 bar.

A further advantageous of the process of the present invention is thatthe cyclic acetals can easily be separated from the reaction mixture.The cyclic acetal, especially the trioxane can be separated from thereaction mixture by distillation in a high purity grade. Especially incase aprotic compounds (such as sulfolane) having a boiling point higherthan about 20° C. above the boiling point of the cyclic acetals is usedthe formed cyclic acetals can simply be distilled off. In case sulfolaneis used as the aprotic compound the formed trioxane can be distilled offwithout the formation of an azeotrope of sulfolane with trioxane. Theprocess of the invention can be carried out batch wise or as acontinuous process.

In a preferred embodiment the process is carried out as a continuousprocess wherein the polyoxymethylene polymer is continuously fed to theliquid medium comprising the catalyst and wherein the cyclic acetals,e.g. the trioxane, is continuously separated (isolated) by separationmethods such as distillation.

The process of the invention leads to an extremely high conversion ofthe polyoxymethylene polymer to the desired cyclic acetals.

According to a preferred embodiment the final conversion of thepolyoxymethylene polymer to the cyclic acetal is greater than 10%, basedon initial polyoxymethylene polymer.

The final conversion refers to the conversion of the polyoxymethylenepolymer into the cyclic acetals in the liquid system. The finalconversion corresponds to the maximum conversion achieved in the liquidsystem.

The final conversion of the polyoxymethylene polymer to the cyclicacetals can be calculated by dividing the amount of cyclic acetals(expressed in wt.-%, based on the total weight of the reaction mixture)in the reaction mixture at the end of the reaction divided by the amountof polyoxymethylene polymer (expressed in wt.-%, based on the totalweight of the reaction mixture) at the beginning of the reaction at t=0.

For example the final conversion of the polyoxymethylene polymer totrioxane can be calculated as:

Final conversion=(amount of trioxane in the reaction mixture expressedin weight-% at the end of the reaction)/(amount of polyoxymethylenepolymer in the reaction mixture expressed in weight-% at t=0 [initialamount of polyoxymethylene polymer in the reaction mixture])

According to a further preferred embodiment of the process of theinvention the final conversion of the polyoxymethylene polymer into thecyclic acetals, preferably trioxane and/or tetroxane, is higher than12%, preferably higher than 14%, more preferably higher than 16%,further preferably higher than 20%, especially higher than 30%,particularly higher than 50%, for example higher than 80% or higher than90%.

According to a further preferred embodiment of the process of theinvention the conversion of the polyoxymethylene polymer into the cyclicacetals, preferably trioxane and/or tetroxane, is higher than 12%,preferably higher than 14%, more preferably higher than 16%, furtherpreferably higher than 20%, especially higher than 30%, particularlyhigher than 50%, for example higher than 80% or higher than 90%.

As described above, the process of the present disclosure converts apolyoxymethylene polymer into one or more cyclic acetals. The resultingcyclic acetals can be used in numerous and diverse applications. In oneembodiment, for instance, the cyclic acetals produced through theprocess may then be used to produce a thermoplastic polymer, such as apolyoxymethylene polymer. Thus, in one embodiment, reclaimedpolyoxymethylene polymers are converted into a cyclic acetal which isthen used as a monomer to produce further amounts of a polyoxymethylenepolymer.

The oxymethylene polymer production process may comprise any suitableprocess for producing oxymethylene homopolymers and/or copolymers. Thepolymer production process, for instance, may comprise an anionicpolymerization process or a cationic polymerization process. The processfor producing the oxymethylene polymer may comprise a heterogeneousprocess where the polymer precipitates in a liquid, may comprise ahomogeneous process such as a bulk polymerization process that forms amolten polymer or may be a polymer process that includes both aheterogeneous phase and a homogeneous phase.

For the preparation of oxymethylene polymers, a monomer that forms—CH₂—O— units or a mixture of different monomers, are reacted in thepresence of an initiator. Examples of monomers that form —CH₂O— unitsare formaldehyde or its cyclic oligomers, such as1,3,5-trioxane(trioxane) or 1,3,5,7-tetraoxocane.

The oxymethylene polymers are generally unbranched linear polymers whichgenerally contain at least 80 mol %, preferably at least 90 mol %, inparticular at least 95 mol %, of oxymethylene units (—CH₂—O—). Alongsidethese, the oxymethylene polymers contain —(CH₂)x-O— units, where x canassume the values from 2 to 25. Small amounts of branching agents can beused if desired. Examples of branching agents used are alcohols whosefunctionality is three or higher, or their derivatives, preferably tri-to hexahydric alcohols or their derivatives. Preferred derivatives areformulas in which, respectively, two OH groups have been reacted withformaldehyde, other branching agents include monofunctional and/orpolyfunctional glycidyl compounds, such as glycidyl ethers. The amountof branching agents is usually not more than 1% by weight, based on thetotal amount of monomer used for the preparation of the oxymethylenepolymers, preferably not more than 0.3% by weight.

Oxymethylene polymers can also contain hydroxyalkylene end groups—O—(CH₂)_(x)—OH, alongside methoxy end groups, where x can assume thevalues from 2 to 25. These polymers can be prepared by carrying out thepolymerization in the presence of diols of the general formulaHO—(CH₂)_(x)—OH, where x can assume the values from 2 to 25. Thepolymerization in the presence of the diols leads, via chain transfer,to polymers having hydroxyalkylene end groups. The concentration of thediols in the reaction mixture depends on the percentage of the endgroups intended to be present in the form of —O—(CH₂)_(x)—OH, and isfrom 10 ppm by weight to 2 percent by weight.

The molecular weights of these polymers, expressed via the volume meltindex MVR, can be adjusted within a wide range. The polymers typicallyhave repeat structural units of the formula —(CH₂—O—)_(n)—, where nindicates the average degree of polymerization (number average) andpreferably varies in the range from 100 to 10 000, in particular from500 to 4000.

Oxymethylene polymers can be prepared in which at least 80%, preferablyat least 90%, particularly preferably at least 95%, of all of the endgroups are alkyl ether groups, in particular methoxy or ethoxy groups.

Comonomers that may be used to produce oxymethylene copolymers includingcyclic ethers or cyclic formals. Examples include, for instance,1,3-dioxolane, diethylene glycol formal, 1,4-butanediol formal, ethyleneoxide, propylene 1,2-oxide, butylene 1,2-oxide, butylene 1,3-oxide, 1,3dioxane, 1,3,6-trioxocane, and the like. In general, one or more of theabove comonomers may be present in an amount from about 0.1 to about 20mol %, such as from about 0.2 to about 10 mol %, based on the amount oftrioxane.

The molecular weight of the resultant homo- and copolymers can beadjusted via use of acetals of formaldehyde (chain transfer agents).These also lead to production of etherified end groups of the polymers,and a separate reaction with capping reagents can therefore be omitted.Chain transfer agents used are monomeric or oligomeric acetals offormaldehyde. Preferred chain transfer agents are compounds of theformula I

R¹—(O—(CH₂)_(q)—O—R²  (I),

in which R¹ and R², independently of one another, are monovalent organicradicals, preferably alkyl radicals, such as butyl, propyl, ethyl, andin particular methyl, and q is a whole number from 1 to 50.

Particularly preferred chain transfer agents are compounds of theformula I, in which q=1, very particularly preferably methylal. Theamounts used of the chain transfer agents are usually up to 5000 ppm,preferably from 100 to 3000 ppm, based on the monomer (mixture).

The initiators used can comprise the cationic initiators usually used inthe preparation of oxymethylene homo- and copolymers. Examples of theseare protic acids, e.g. fluorinated or chlorinated alkyl- andarylsulfonic acids, such as trifluoromethanesulfonic acid,trifluoromethanesulfonic anhydride, or Lewis acids, such as stannictetrachloride, arsenic pentafluoride, phosphorus pentafluoride, andboron trifluoride, and also their complex compounds, e.g. borontrifluoride etherate, and carbocation sources, such as triphenylmethylhexafluorophosphate.

In one embodiment, the initiator for cationic polymerization is anisopoly acid or a heteropolyacid or an acid salt thereof which may bedissolved in an alkyl ester of a polybasic carboxylic acid.

The heteropoly acid is a generic term for polyacids formed by thecondensation of different kinds of oxo acids through dehydration andcontains a mono- or poly-nuclear complex ion wherein a hetero element ispresent in the center and the oxo acid residues are condensed throughoxygen atoms. Such a heteropoly acid is represented by formula (1):

H_(x)[M_(m)M′_(n)O_(z) ].yH₂O  (1)

whereinM represents an element selected from the group consisting of P, Si, Ge,Sn, As, Sb, U, Mn, Re, Cu, Ni, Ti, Co, Fe, Cr, Th and Ce,M′ represents an element selected from the group consisting of W, Mo, Vand Nb,m is 1 to 10,n is 6 to 40,z is 10 to 100,x is an integer of 1 or above, andy is 0 to 50.

In another embodiment, the initiator for cationic polymerizationcomprises at least one protic acid and at least one salt of a proticacid, wherein said at least one protic acid is sulfuric acid,tetrafluoroboric acid, perchloric acid, fluorinated alkyl sulfonic acid,chlorinated alkyl sulfonic acid or aryl sulfonic acid, and wherein saidsalt of protic acid is an alkali metal or alkaline earth metal salt ofprotic acid and/or a substituted ammonium salt of protic acid, thecations of the ammonium salt having the general formula (I)

where R¹-R⁴ are independently hydrogen, an alkyl group or an aryl group.

Particular preference is given to substituted ammonium ions having thegeneral formula (I)

where R¹ to R⁴ are independently hydrogen, an alkyl group such asmethyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl or an aryl groupsuch as phenyl or 4-methoxypheny.

In order to terminate the polymerization, the reaction mixture, whichstill comprises unconverted monomers and/or byproducts, such as trioxaneand formaldehyde, alongside polymer, is brought into contact withdeactivators. These can be added in bulk form or a form diluted with aninert solvent to the polymerization mixture. The result is rapid andcomplete deactivation of the active chain ends.

Deactivators that can be used are those compounds which react with theactive chain ends in such a way as to terminate the polymerizationreaction. Examples are the organic bases triethylamine or melamine, andalso the inorganic bases potassium carbonate or sodium acetate. It isalso possible to use very weak organic bases, such as carboxamides, e.g.dimethylformamide. Tertiary bases are particularly preferred, examplesbeing triethylamine and hexamethylmelamine.

The present disclosure may be better understood with respect to thefollowing examples.

EXAMPLES Example 1

10 g of dried Polyoxymethylene Copolymer (with a low Dioxolane content)(TICONA trade name: Hostaform® HS 15) with melt index of 1.5 g/10 minwere dissolved in 90 g of sulfolane at 145° C. with stirring. The clearsolution was added to 20 g sulfolane (at 120° C.) containing 0.4 ml of a10 wt % solution of triflic acid in sulfolane. After the addition wascompleted, the homogeneous solution was cooled to 60° C., neutralizedwith triethylamine and then analyzed. The following composition wasfound:

Trioxane: 7.1 wt % Tetroxane: 0.75 wt % Formaldehyde: 0.4 wt %Methylformate: <20 ppm Example 2

Example 1 was repeated, except that perchloric acid (70 wt % in water)was used for triflic acid:

10 g of dried Polyoxymethylene Copolymer (with a low Dioxolane content)(TICONA trade name: Hostaform® HS 15) with melt index of 1.5 g/10 minwere dissolved in 90 g of sulfolane at 145° C. with stirring. The clearsolution was added to 20 g sulfolane (at 120° C.) containing 1.2 ml of a2 wt % solution of perchloric acid (70 wt % in water) in sulfolane.After the addition was completed, the homogeneous solution was cooled to60° C., neutralized with triethylamine and then analyzed. The followingcomposition was found:

Trioxane: 7.2 wt % Tetroxane: 0.8 wt % Formaldehyde: 0.3 wt %Methylformate: <20 ppm Comparative Example 3

Example 1 was repeated, except that nitrobenzene was used for sulfolaneas a solvent:

10 g of dried Polyoxymethylene Copolymer (with a low Dioxolane content)(TICONA trade name: Hostaform® HS 15) with melt index of 1.5 g/10 minwere dissolved in 90 g of nitrobenzene at 145° C. with stirring. Theclear solution was added to 20 g nitrobenzene (at 120° C.) containing0.4 ml of a 10 wt % solution of triflic acid in sulfolane. After theaddition was completed, the homogeneous solution was cooled to 60° C.,neutralized with triethylamine and then analyzed. The followingcomposition was found:

Trioxane: 6.2 wt % Tetroxane: 0.7 wt % Formaldehyde: 0.7 wt %Methylformate: 0.5 wt %

The GC spectrum also showed a new peak with a retention time beyond thatof nitrobenzene, which was not further analyzed but is believed to be areaction product of nitrobenzene with formaldehyde. Thus, nitrobenzeneis not stable under reaction conditions, produces side products(methylformate) and consequently has a lower yield in trioxane.

Example 4

Example 1 was repeated, except that a mixture of Dimethylsulfone (30 g)and Sulfolane (60 g) was used for sulfolane as a solvent:

10 g of dried Polyoxymethylene Copolymer (with a low Dioxolane content)(TICONA trade name: Hostaform® HS 15) with melt index of 1.5 g/10 minwere dissolved in a mixture of Dimethylsulfone (30 g) and Sulfolane (60g) at 145° C. with stirring. The clear solution was added to 20 gsulfolane (at 120° C.) containing 0.4 ml of a 10 wt % solution oftriflic acid in sulfolane. After the addition was completed, thehomogeneous solution was cooled to 60° C., neutralized withtriethylamine and then analyzed. The following composition was found:

Trioxane: 7.1 wt % Tetroxane: 0.6 wt % Formaldehyde: 0.8 wt %Methylformate: 9.4 ppm

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood that aspects of the various embodiments may beinterchanged both in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention sofurther described in such appended claims.

1.-20. (canceled)
 21. A process for the recycling of oxymethylene homo-or copolymers comprising the steps: a) at least partly dissolving anoxymethylene homo- or copolymer in an aprotic compound; b) convertingthe at least partly dissolved oxymethylene homo- or copolymer in thepresence of a catalyst to cyclic acetals; and c) converting the cyclicacetals obtained in step b) optionally together with comonomer(s) tooxymethylene polymers.
 22. A process according to claim 21, wherein atleast 20 wt.-% of the oxymethylene homo- or copolymer is dissolved inthe aprotic compound.
 23. A process according to claim 21, wherein theoxymethylene polymer is at least partly dissolved at a temperaturehigher than 100° C.
 24. A process according to claim 21, wherein theaprotic compound has a boiling point of 120° C. or higher, determined at1 bar.
 25. A process according to claim 21, wherein the oxymethylenepolymer, the aprotic compound and the catalyst form a reaction mixtureand wherein the reaction mixture comprises at least 60 wt.-% of theaprotic compound, wherein the weight is based on the total weight of thereaction mixture.
 26. A process according to claim 21, wherein theaprotic compound comprises a sulfur containing organic compound.
 27. Aprocess according to claim 21, wherein the aprotic compound comprises adipolar nitro-group free compound.
 28. A process according to claim 21,wherein the aprotic compound is represented by formula (I):

wherein n is an integer ranging from 1 to 6, and wherein the ring carbonatoms may optionally be substituted by one or more substituents,selected from C₁-C₈-alkyl which may be branched or unbranched.
 29. Aprocess according to claim 21, wherein the aprotic compound issulfolane.
 30. A process according to claim 21, wherein the aproticcompound is represented by formula (II):

wherein R¹ and R² are independently selected from C₁-C₈-alkyl which maybe branched or unbranched.
 31. A process according to claim 21, whereinthe aprotic compound is represented by formula (III):

wherein n is an integer ranging from 1 to 6, and wherein the ring carbonatoms may optionally be substituted by one or more substituents,selected from C₁-C₈-alkyl which may be branched or unbranched; or theaprotic compound is represented by formula (IV):

wherein R³ and R⁴ are independently selected from C₁-C₈-alkyl which maybe branched or unbranched.
 32. A process according to claim 21, whereinthe conversion to the cyclic acetal is carried out at a temperatureranging from 40° C. to 130° C. and is carried out at a pressure of from10 millibars to 10 bars.
 33. A process according to claim 21, whereinhigher than 30 percent of the oxymethylene homo- or copolymer isconverted into the cyclic acetal.
 34. A process according to claim 21,further comprising the step of converting the cyclic acetals formedduring the process into an oxymethylene polymer.
 35. A process accordingto claim 21, wherein the oxymethylene homo- or copolymer is at leastpartly dissolved in the aprotic compound while being heated and whereinthe resulting liquid medium is cooled prior to being contacted with thecatalyst.